Media for recording information using high energy density beams, such as laser beams, have recently been developed and become of practical use. Such media are generally referred to as optical discs and can be used as a video disc, an audio disc, and as files for a large volume of still images, as well as for disc memory for computers containing a large volume of information.
An optical disc essentially contains a transparent disc substrate composed of plastic or glass and a recording layer provided on the substrate composed of a metal such as Bi, Sn, In, or Te, or a semi-metal. A subbing layer composed of a polymeric substance is generally provided between the substrate and the recording layer to improve adhesion between the substrate and the recording layer, as well as to improve sensitivity of the optical disc.
Information can be written on an optical disc, for example, by irradiating a laser beam on the optical disc. That is, the irradiated area of the recording layer absorbs the laser beam, the temperature on the irradiated area increases higher, generating physical or chemical changes as well as optical characteristic changes of the recording layer, whereby information is recorded. Information can be read, for example, by irradiating a laser beam on an optical disc. It can also be reproduced, e.g., by detecting reflected light or transmitted light corresponding to a change of optical characteristics of the recording layer.
In the optical information recording medium, a laser beam focused to about a 1 .mu.m diameter is irradiated on the surface of a disc to write and read information. If dirt is adhered on a recording layer or if a recording layer is scratched while the disc is handled, problems such as bit errors or dropouts readily occur. In this connection, a so-called "air sandwich structure", wherein two transparent disc substrates, each carrying a recording layer, are engaged through a spacer or a convex portion provided on the substrate for positioning two substrates carrying the recording layers facing each other in spaced relation has been proposed, as described in, for example, U.S. Pat. Nos. 4,074,282, 4,264,911 and 4,353,767.
Further, a so-called "directly adhered structure" wherein two transparent disc substrates carrying at least one recording layer are adhered directly with an adhesive layer so that the recording layer is provided inside surface of the substrate has been proposed, as described in, for example, Japanese Patent Application (OPI) Nos. 130243/82 and 133532/82 (the term "OPI" as used herein refers to a "published unexamined application").
However, when above-described discs are transported, used, and stored under different circumstances, various problems occur with changes in temperature, humidity, or air-pressures, due to use at different altitudes.
Particularly in a disc having an air sandwich structure, there are many problems. For instance, pressure differences occur between the inside space (air cavity) filled with inactive gas and the outside air, according to changes in temperatures or changes in air pressure according to the altitudes at which discs are used. As a result, the inside filled gas contracts or expands, thereby deforming substrates in some cases. Deformation of substrates easily leads to incapability of focus-servo devices and extreme deformation leads to peeling off of the adhered area of the substrates or to otherwise damaging the substrates.
Furthermore, due to the difference in the thermal expansion coefficient between a substrate and a spacer, shearing strain is added to an adhesive layer provided between the substrate and the spacer, and thus the substrate tends to be peeled off from the spacer. Still further, due to stress added to a substrate, the substrate is easily damaged. Even due to slight stress added to a substrate, double refraction may occur in the substrate, thereby deteriorating recording and playback of signals.
In a disc having directly adhered structure, if the thermal expansion coefficients of two substrates are different from each other, or if only one of two substrates is heated, the disc tends to deform or adhered areas of the substrates readily peel off.
When the above mentioned information recording disc is transported or used, or when the disc falls by error, as a result of impact, it often happens that substrates are damaged or broken, or the adhered areas of the substrates peel off, and double refraction due to stress occurs.
Very desirably, even under various conditions of temperature, humidity, and altitude, and even under rough handling conditions, including impacts, a disc should not be deformed, a substrate should not be damaged, and adhered areas should not be peeled off. Also, during use of a disc, out of focus conditions should not be caused by deterioration of the flatness of a disc, and recording and playback of signals should not be affected by double refraction.
To solve the above various problems, a method for providing an air hole in the optical disc having an air sandwich structure has been proposed, as described in Japanese Utility Model Application (OPI) Nos. 11613/80, 149535/82, 72765/83, 105041/83 and 105042/83.
This method can prevent the deformation of a disc caused due to a pressure difference between the inside space (air cavity) and the ambient air, but cannot, however, prevent peeling off of an adhered area due to a difference in the thermal expansion coefficient between a substrate and a spacer, nor the occurrence of double refraction due to stress to a substrate. Furthermore, this method creates serious problems that water vapor in the air enters into the inside space of a disc through such an air hole and deteriorates the recording layer, and similarly outside dirt and dust enter thereinto, generating errors.
Another method for using a spacer having the same thermal expansion coefficient with that of a substrate has been propsoed, as described in Japanese Patent Application (OPI) No. 143446/83. This method, however, cannot prevent deformation of a disc due to a pressure difference between the inside space and the outside ambient air, and selection of a spacer having a particular thermal expansion coefficient may be costly.
As another solution, a method for using a soft spacer as a center spacer has been proposed, as described in Japanese Patent Application (OPI) No. 71145/84. This method, however, cannot prevent deformation of a disc due to a pressure difference between the inside space and the outside ambient air, and use of a particular spacer is costly, too.
Another method involving making flexible a protective substrate disc which has no recording layer thereon and is positioned facing the other substrate disc carrying recording layer in air sandwich structure of a disc has been proposed, as described in Japanese Patent Application (OPI) No. 171048/84. In accordance with this method, peeling off of an adhered area caused due to pressure difference between the inside space and the outside ambient air can be prevented, but there is a disadvantage in that only one recording layer can be provided, and therefore the volume of information that can be recorded is reduced.
The present invention is to provide an optical information recording medium to solve the above problems.